1. Field of the Invention
The present invention relates to a field-effect transistor and, more particularly, to a field-effect transistor such as a metal-semiconductor contact gate field-effect transistor, a p-n junction field-effect transistor, or the like, that is useful as a high-efficiency, high-frequency amplifying element.
2. Description of the Related Art
Metal-semiconductor contact gate field-effect transistors (MESFETs) fabricated from gallium arsenide (GaAs) are often used for high-frequency amplifiers because of their high-frequency performance and efficiency.
FIG. 1 shows a typical structure of a recessed-gate GaAs MESFET, for example. The GaAs MESFET 100 shown here comprises a buffer layer 12, an active layer 14, and a contact layer 16, formed successively on a semi-insulating GaAs substrate 10 using an epitaxial growth technique. The buffer layer 12 is not doped with impurities and acts as an insulating layer. The active layer 14 has a relatively low impurity concentration and forms a high-resistivity layer. On the other hand, the contact layer 16 has a higher impurity concentration than the active layer 14, and therefore forms a low-resistivity layer. The impurity concentration is controlled at a uniform level across each of the active layer 14 and the contact layer 16. A portion of the contact layer 16 is removed by etching, to form a recess 18 where a portion of the underlying active layer 14 is exposed. A gate 20 is formed on the exposed portion of the active layer 14, and the gate 20 and the active layer 14 form a Schottky contact. On the contact layer 16, a source electrode 22 and a drain electrode 24 are formed on both sides of the gate electrode 20, each forming an ohmic contact to the contact layer 16.
In the above described GaAs MESFET 100, when a positive voltage (Vds) is applied to the drain electrode 24 with respect to the source electrode 22, current flows through the active layer 14 from the drain electrode 24 to the source electrode 22. Because the gate electrode 20 and the active layer 14 form a Schottky barrier therebetween, the depth of the depletion layer can be varied by controlling the gate voltage (Vgs), as shown in FIG. 2, as a result of which the cross-sectional area of the channel varies and the drain-source current (Ids) can thus be controlled. FIG. 2 shows in schematic form the current-voltage curves for various gate voltages.
In this type of MESFET, it has been difficult to achieve a reduction in the ON resistance and an increase in the gate breakdown voltage at the same time. The reasons for this and methods for improvement will be described below.
(A) ON resistance
The ON resistance refers to the resistance between source and drain in the linear region of the drain-source voltage versus drain-source current characteristic curves shown in FIG. 2. The ON resistance Ron is expressed by the following equation (see FIG. 1). EQU Ron=Rch+2R1+2R2+2Rco
where Rch is the channel resistance, R1 is the resistance of the recess, R2 is the resistance of the contact layer 16, and Rco is the ohmic contact resistance.
By reducing the ON resistance, power loss during the ON period of the MESFET can be reduced. The following two measures are effective in reducing the ON resistance.
A1: Reduce the channel resistance Rch by increasing the thickness and the doping level of the active layer.
A2: Reduce the ohmic contact resistance Rco and contact layer resistance R2 by increasing the thickness and the doping level of the contact layer.
(B) Gate breakdown voltage
When a negative voltage is applied between the gate and drain of a MESFET, the current flowing between the gate and drain suddenly begins to increase rapidly when a certain voltage is reached. The gate-drain voltage at which this phenomenon occurs is the gate breakdown voltage. This phenomenon will be explained below with reference to FIG. 3.
When the negative voltage (Vg) applied to the gate electrode 20 is increased, the depletion layer A extends from one end of the gate electrode 20 toward the drain electrode 24. When the voltage is further increased until the end of the depletion layer A reaches the interface with the contact layer 16, the depletion layer ceases to extend, and the electric field begins to concentrate at a lower end portion of the gate electrode 20. This is because the electric field is easier to form in the high-resistivity active layer 14 than in the low-resistivity contact layer. When the field density in this field concentration region B exceeds the breakdown field density of the active layer, the current flowing between the gate and drain suddenly begins to increase rapidly. The gate-drain voltage that causes this sudden increase is the gate breakdown voltage.
By increasing the gate breakdown voltage, power loss during the OFF period of the MESFET can be reduced. The following three measures are effective in increasing the gate breakdown voltage.
B1: Reduce the doping level of the active layer.
B2: Reduce the thickness and the doping level of the contact layer.
B3: Increase the recess depth.
As described above, reducing the ON resistance and increasing the gate breakdown voltage are contradicting requirements (A1 vs. B1, A2 vs. B2), and it is difficult to achieve both at the same time.